The present invention relates to offset cancellation in general and to DC offset cancellation in mobile communication systems in particular.
Many transmitters transmit digital information values that are generated in base band. The base band digital values are modulated onto a carrier high frequency signal and the combined signal is amplified before its transmission. The base band values may be complex values having real and imaginary components which are traditionally referred to as I and Q components, respectively. In the modulation of the base band signal and amplification of the modulation signal, inaccuracies are introduced. These inaccuracies may cause the transmitter to interfere with signals on carrier frequencies allocated to other transmitters and therefore should be at least partially canceled by the transmitter.
One source of inaccuracy is the IQ modulator and demodulator, which both suffer from a distortion mechanism called xe2x80x9clocal oscillator carrier feedthroughxe2x80x9d and xe2x80x9cDC offsetxe2x80x9d. For example, the output of the demodulator can be modeled as:
S(t)=I(t)cos(xcfx89t)xe2x88x92Q(t)sin(xcfx89t)+LO(t)xe2x80x83xe2x80x83Equation 1
where:
LO(t)=A cos(xcfx89t+xcfx86).xe2x80x83xe2x80x83Equation 2
The DC offset mechanism is generally due to LO(t), which is caused due to leakage of the signal of a local oscillator (used for carrier modulation) into the demodulator output. The same problem occurs at the modulator side.